1. Field of the Invention
The present invention relates to an image processing method and system for converting multi-gradated image data into binary image data having a resolution different from that of the multi-graded image data.
2. Description of the Related Art
A typical image forming system is capable of converting original image data, which is read by a scanner or the like, into data which is compatible with an image output unit such as a printer. Some of the conventional image forming systems are also designed to output original image data after changing its resolution.
Published Japanese patent application no. 155761/1990, for example, describes an image processing method for outputting original image data after changing its resolution from 300 dpi to 600 dpi. Also, published Japanese patent application no. 231861/1991 describes a printing method under which a second dot pattern is complementarily struck against a first dot pattern to create a printed image having higher resolution.
Both of these conventional methods are intended to reproduce the original image with precision by increasing the resolution of the image data. However, the higher the set resolution, the longer the image processing time becomes, thus making high-speed printing impossible.
When using a printer incapable of regeneratively displaying multi-gradation image data, that is, a printer which is incapable of controlling the gradation of each dot or pixel of the image, a binarization process is employed to change the gradation of each pixel of a multi-gradated original image to a binary gradation. Although there are various methods of binarizing multi-gradated image data, an error diffusion method and an average error minimization method typically provide excellent image quality.
As is well known in the art, the error diffusion method is used to add a quantitative error, which arises when a pixel is binarized, to peripheral pixels that have not yet been subjected to binarization, by diffusion. On the other hand, the average error minimization method, which is also well known, corrects the data value of a selected pixel with the weighted mean values of the quantitative errors attributed to binarized pixels surrounding the selected pixel.
The error diffusion method and the average error minimization method are different only because of different timing at which the error diffusing operation is performed and hence, are theoretically equivalent. Published Japanese Patent application no. 284173.backslash.1989, for example, describes one of the specific examples of the binarization process using the error diffusion method.